The present invention relates generally to an optical lens, a condenser lens composed of a plurality of optical lens including this optical lens, an optical pickup including this condenser lens, and an optical recording/reproducing apparatus (inclusive of a magneto-optical recording/reproducing apparatus) including this optical pickup, and more particularly to an optical lens formed of a material having a high refractive index in a wavelength region of visible light, thereby increasing a numerical aperture to make it suitable for a near-field optical recording/reproducing system for performing recording and/or reproduction on an optical recording medium.
An optical recording medium typically including a compact disc (CD), mini disc (MD), and digital video disc (DVD) is widely used as a storage medium for audio information, video information, data, program, and so forth (the term of “optical recording medium” mentioned in this specification is meant to also include a magneto-optical recording medium).
However, higher sound quality, higher image quality, longer operation time, and higher capacity are required in audio information, video information, data, program, and so forth, and it is therefore desired to develop a higher-capacity optical recording medium and an optical recording/reproducing apparatus for performing recording/reproduction on such a higher-capacity optical recording medium (the term of “optical recording/reproducing apparatus” mentioned in this specification is meant to also include a magneto-optical recording/reproducing apparatus).
In a conventional optical recording/reproducing apparatus designed to meet the above requirement, the wavelength of light to be emitted from a light source (e.g., semiconductor laser) is shortened or the numerical aperture of a condenser lens is increased to thereby reduce the diameter of a beam spot formed by convergence of light through the condenser lens.
Concerning the semiconductor laser, a GaN semiconductor laser having an oscillation wavelength of 400 nm band shortened from 635 nm as the oscillation wavelength of a conventional red laser is toward practical use, thereby reducing the diameter of the beam spot.
For shorter oscillation wavelengths, a far-ultraviolet solid-state laser, UW-1010 manufactured by Sony Corporation, capable of continuously oscillating light having a single wavelength of 266 nm is on the market, thus further reducing the diameter of the beam spot. Other examples under research and development include a frequency-doubled laser (266 nm band) from a Nd:YAG laser, a diamond laser (235 nm band), and a frequency-doubled laser (202 nm band) from a GaN laser.
Further, a near-field optical recording/reproducing system is under study, wherein an optical lens having a large numerical aperture such as a solid immersion lens (SIL) typically is used to realize a condenser lens having a numerical aperture of more than 1, and the objective surface of the condenser lens is positioned close to a recording medium with a spacing substantially equal to the wavelength of light from a light source to thereby perform recording/reproduction. Non-patent Literature 1 shown below is a literature on the near-field optical recording/reproducing system using the solid immersion lens.
[Non-patent Literature 1]
I. Ichimura et. al., “Near-Field Phase-Change Optical Recording of 1.36 Numerical Aperture”, Jpn. J. Appl. Phys. Vol. 39, 962-967 (2000).
In this near-field optical recording/reproducing system, it is important how the distance between the optical recording medium and the condenser lens is maintained in an optical contact condition. Further, since the diameter of a light beam emitted from the light source and entering the condenser lens is small and the distance between the optical recording medium and the condenser lens is very small, the condenser lens is greatly limited in shape.
FIG. 1 is a schematic sectional view of an optical system in the case of recording information by using the near-field optical recording/reproducing system. The configuration of the optical system shown in FIG. 1 will be hereinafter described in detail in the description of a preferred embodiment of the present invention. As shown in FIG. 1, a first optical lens 11 is opposed to an optical recording medium 30, and a second optical lens 12 is opposed to the first optical lens 11. In other words, the first and second optical lenses 11 and 12 are arranged in this order from the optical recording medium 30 side. The first optical lens 11 is provided by a super-semispherical optical lens formed of glass (SiO2) having a refractive index of n=1.5 (a specific configuration of the super-semispherical optical lens will be hereinafter described). These optical lenses 11 and 12 constitute a near-field condenser lens 13. In this example, the numerical aperture of the near-field condenser lens 13 is calculated as 1.25[=n×sin(tan−1n)=1.5×sin(tan−11.5)].
In the first optical lens (super-semispherical optical lens) 11, the relation of t=r(1+1/n) is given where r is the radius of curvature of the optical lens 11, n is the refractive index of the optical lens 11, and t is the thickness of the optical lens 11. Letting WD denote the distance between the second optical lens 12 and the optical recording medium 30 which distance is determined by the numerical aperture of the second optical lens 12, the condition of t=r(1+1/n)=1.667r<WD must be satisfied. Accordingly, in suitably and easily realizing the distance between the first optical lens 11 and the second optical lens 12, it is necessary to minimize the radius of curvature r or to maximize the refractive index n of the optical lens 11.
However, the radius of curvature r of the first optical lens 11 cannot be reduced to less than 1 mm from the viewpoint of the assembly accuracy of an optical pickup including this optical lens. In general, a condenser lens in a near-field optical recording/reproducing system is composed of first and second optical lenses arranged in this order from the side of an optical recording medium, and a numerical aperture of more than 1 for the condenser lens is realized by the combination of these two optical lenses. The assembly accuracy of these two optical lenses is required to become higher with an increase in the numerical aperture, and this accuracy must be maintained against changes in environment. Therefore, if the radius of curvature of the first optical lens is too small, the required assembly accuracy of the near-field condenser lens cannot be realized.
Further, the refractive index of the first optical lens formed of glass (SiO2) is limited to about 1.5 at the maximum, so that the thickness of the first optical lens cannot be reduced below a certain limit.
On the other hand, in realizing a higher density in the near-field optical recording/reproducing system, a beam spot to be formed by the condenser lens must be reduced by shortening the wavelength of light to be emitted from a light source or by increasing the numerical aperture of the condenser lens as in a conventional normal optical recording system. The area of the beam spot is inversely proportional to the square of the numerical aperture of the condenser lens. It is therefore effective to increase the numerical aperture of the condenser lens in realizing a higher density in the near-field optical recording/reproducing system.
For example, in the case that the first optical lens is a super-semispherical optical lens as in FIG. 1, the numerical aperture NA of the near-field condenser lens is given by NA=n×sin(tan−1n) where n is the refractive index of the first optical lens. Conventionally, both the first and second optical lenses are formed of glass (SiO2), so that the refractive index of the first optical lens is limited to 1.5 at the maximum in a wavelength region of visible light. Accordingly, in the case that the first optical lens is a super-semispherical optical lens formed of glass, the maximum numerical aperture NA of the near-field condenser lens is given by NA=1.5×sin(tan−11.5)=1.25. Accordingly, there is a limit to increasing the density by using the near-field condenser lens formed of glass.